Rheumatoid arthritis (RA) is an inflammatory autoimmune disease that leads to destruction of bone and cartilage and is associated with increased morbidity and mortality. The mechanism responsible for the persistent inflammation of the synovium is not clear. We were the first to demonstrate that expression of the cyclin dependent kinase (CDK) inhibitor p21(WAF1/CIP1) is reduced in synovial tissue from RA patients compared to osteoarthritis patients. Although one of the major functions of p21 is to arrest the cell cycle, we recently uncovered a novel role for p21 in the suppression of the inflammatory response in macrophages. Macrophages lacking p21 (p21-/-) secrete elevated levels of pro-inflammatory cytokines and display increased markers of activation following ligation by toll like receptor (TLR) agonists. Further, CDK2, a target for p21, is constitutively active in p21-/- macrophages while CDK4 is induced in Wt and p21-/- macrophages following TLR stimulation despite the fact that these cells are terminally differentiated and are withdrawn from the cell cycle. Mice lacking p21 exhibit an enhanced and sustained development of experimental inflammatory arthritis which is associated with markedly increased numbers of macrophages and severe articular destruction. Further, administration of a p21 peptide mimetic suppresses activation of macrophages and prevents development of experimental arthritis. Mechanistically, the increased activation in p21-/- macrophages is associated with the reduction in the activity of the serine/threonine kinase Akt and an increase in the activation of the mitogen-activated protein kinase, p38. Treatment with the p21-peptide mimetic restores Akt and reduces p38 activity in macrophages. Based on these data, we hypothesize that decreased expression of p21 in macrophages leads to the enhanced production of pro-inflammatory molecules mediated by reduced levels of active Akt and increased phosphorylated Ask1 and p38, independent of CDK activation. Taken together, the data anticipated from these studies will be the first to reveal that in macrophages, p21 is apical in determining the fate of the TLR intracellular signaling cascade. Thus, the elucidation of the molecular mechanism governing p21 in macrophages is crucial for understanding the development and persistence of RA and for potential new therapeutic targets for ameliorating RA. PUBLIC HEALTH RELEVANCE: Inflammatory diseases, including rheumatoid arthritis, are responsible for significant mortality and morbidity. Because treatments are often ineffective or must be discontinued due to side effects, a greater understanding of the mechanisms involved in the progression of disease is crucial to the development of more efficacious treatments. Analysis of synovial tissue sections from rheumatoid arthritis patients revealed a decrease in expression of a cell cycle inhibitor protein, p21. Further studies using a mouse model of arthritis showed that increasing the level of p21 reduces the severity of disease. These data indicate that p21 may be an inhibitor of arthritis and other inflammatory diseases. Therefore, our goal with these studies is to uncover the mechanism for p21-mediated inhibition of inflammation.